This is an open-label, phase I, repeated-dose, dose-escalation study in patients of good performance status with metastatic breast cancer. Patients will be treated with one daily three-hour infusion of PNU-214936 for four consecutive days to evaluate for adverse events, to assess immunologic response to treatment and to measure pharmacokinetic parameters. DLTs and MTD of PNU-214936 will be evaluated in this initial treatment cycle course. Dose escalations will proceed in successive patients based on a continuously adjusted statistical model that takes into account baseline anti-SEA levels as well as the occurrence of DLT in prior patients. To ensure safety, the dose in a new patient will never be more than double the highest safe dose given to any prior patient. All patients will be evaluated on Days 11 and 28 following the first infusion and those patients who demonstrate either a tumor response or stable disease at Day 28 may have a second 4-day treatment cycle with PNU-214936. Patients will be further evaluated at Day 56, 84 and every 56 days thereafter until evidence of tumor progression.